PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "Scott Murchie, 2007-08-01; edited, 2007-10-03; edited, 2014-09-08; edited, MReid 2014-09-11; edited, JWard 2014-09-17." OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "MESS" INSTRUMENT_ID = "MDIS-NAC" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "MERCURY DUAL IMAGING SYSTEM NARROW ANGLE CAMERA" INSTRUMENT_TYPE = "FRAMING CAMERA" INSTRUMENT_DESC = " Instrument Overview =================== The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft provides critical measurements tracing Mercury's origin and evolution. MDIS consists of a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC). The NAC is a 1.5 degree field-of- view (FOV) off-axis reflector, coaligned with the WAC, a four-element refractor with a 10.5 degree FOV and 12-color filter wheel. The focal plane electronics of each camera are identical and use a 1024x1024 Atmel (Thomson) TH7888A charge-coupled device (CCD) detector. Only one camera operates at a time, allowing them to share a common set of control electronics. The NAC and the WAC are mounted on a pivoting platform that provides a 90 degree field of regard, extending 40 degrees sunward and 50 degrees anti-sunward from the spacecraft +Z-axis, the boresight direction of most of MESSENGER's instruments. Onboard data compression provides capabilities for pixel-binning, remapping of 12-bit data into 8 bits, and lossless or lossy compression. Scientific Objectives ===================== The MESSENGER spacecraft made three flybys of Mercury, in January 2008, October 2008, and September 2009, during which regions unexplored by Mariner 10 were imaged by MDIS. In March 2011, the spacecraft began the orbital phase of its mission, which including the Primary Mission and two Extended Missions continues through spacecraft impact in March or April 2015. The primary imaging objectives during the flybys were: (1) acquisition of near-global NAC coverage at about 500 m/pixel, (2) 11-color multispectral WAC mapping at about 2 km/pixel, and (3) high-resolution WAC and NAC imaging of select regions. During the initial 1-year segment of the orbital phase of the mission, the MDIS observing shifted to acquisition of additional datasets: (4) a nadir-looking monochrome (750-nm) global mosaic at moderate solar incidence angles (55-75 degrees) and 250 m/pixel or better sampling; (5) a 20-degree off-nadir monochrome mosaic to complement the nadir-looking mosaic for stereo; (6) repeated monochrome mosaics of the south polar regions to identify regions which remain permanently shadowed, using the WAC 750-nm filter one solar day and the NAC the next solar day; (7) WAC multispectral mapping at 1 km/pixel in 8 colors at low incidence angles; (8) high-resolution (20-50 m/pixel) NAC image strips across features representative of major geologic units and structures; (9) 3-color WAC imaging of select regions at low incidence angles, commonly at pixel scales near 400 m/pixel; (10) WAC color photometry to support derivation of a photometric function for the 8-color map, repeatedly covering representative areas near the Rembrandt and Beethoven basins at a wide variety of incidence, emission, and phase angles, initially using the same 8-color filter set as for the global 8-color map; (11) 3 weekly sets of 2x1 frame WAC 750-nm image mosaics showing the entire limb of Mercury, to help define the low-order global shape model for Mercury; and (12) images of star fields to calibrate NAC and WAC focal length and geometric distortion, the orientation of the cameras on the pivot, the pivot angle, and the orientation of the base of the pivot and how it changes with time as a result of thermal distortions. For the extended missions, several additional data sets were acquired: (13) a nadir-looking monochrome (750-nm) 'albedo' global mosaic at low incidence angles and 250 m/pixel or better sampling; (14) a 20-degree off-nadir monochrome mosaic to complement the nadir-looking low-incidence angle mosaic for stereo with fewer gaps due to shadows in areas with steep topography; (15) a nadir-looking monochrome (750-nm) 'high incidence from the west' global mosaic with incidence angles near 80 degrees and solar azimuth to the west, at 250 m/pixel or better sampling; (16) a nadir-looking monochrome (750-nm) 'high incidence from the east' global mosaic with incidence angles near 80 degrees and solar azimuth to the east, at 250 m/pixel or better sampling; (17) a nadir-looking 3-color mapping of northern and equatorial latitudes at an improved spatial sampling compared to the 8-color map, about 400 m/pixel; (18) a WAC 750-nm monochrome mosaic of the south polar regions to identify regions which remain permanently shadowed; (19) NAC 'ride-along' imaging of the northern hemisphere to acquire a broad sampling of different terrains at <= 20 m/pixel; (20) repeated WAC color imaging of high southern latitudes to track time variations in radiometric calibration; (21) off-nadir WAC 5-color mapping of the northern plains at a near- constant low phase angle near 30 degrees; (20) WAC clear-filter and color imaging of permanently shadowed craters, to determine the spatial distribution of low- and high-reflectance deposits of frozen volatiles; (21) 11-color targets using the full set of WAC spectral filters covering regions of interest for their spectral variations; (22) 3-color WAC targets at 2 to 5 phase angles within single orbits or groups of two orbits, to measure spatial differences in photometric properties; and (23) movies using the WAC 750-nm filter pointed into the ram direction to capture a view of flying over Mercury's surface at a low altitude. For monochrome global maps, data are acquired with the NAC for southern latitudes when spacecraft altitude is high, and with the WAC at lower altitudes over the northern hemisphere. This two-camera strategy results in near-uniform global coverage with an average spatial resolution of about 200 m/pixel. Off-nadir mosaics for stereo were acquired under nearly identical lighting geometries to the nadir map to facilitate automated stereo matching. Subsystems ========== MDIS consists of two cameras, a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC), coaligned on a common pivot platform. The passively cooled detectors in each camera are thermally tied to the complex thermal system. This allows the detectors to be maintained within their operating temperature of -10C, except during the hottest portion of the orbit at Mercury. The pivot platform provides an added degree of freedom to point the dual cameras with minimal impact on the spacecraft. MDIS is mounted in MESSENGER's payload adaptor ring and faces in the nadir direction in Mercury orbit along the spacecraft +z axis, with the pivot aligned in the y-z plane where -y is toward the sun. Specifications of the two cameras are given below: Narrow Angle Wide Angle Field of view 1.5 x 1.5 degrees 10.5 x 10.5 degrees Pivot range Both -40 deg (toward Sun) to +50 deg (away from Sun) Exposure time 1 ms to 9989 ms 1 ms to 9989 ms Frame transfer time 3.4 ms 3.4 ms Image readout time 1 s 1 s Spectral filters 1 filter 12 filters Spectral range 725-783 nm 395-1040 nm Focal length 550 mm 78 mm Collecting area 462 mm**2 48 mm**2 Detector Both TH7888A CCD, 1024x1024 14-micron pixels IFOV 25 microrad 179 microrad Pixel FOV 5.1 m at 200-km altitude 35.8m at 200-km altitude Quantization Both 12 bits/pixel Compression Both have options of lossless, lossy wavelength, 12-to-8 bits, and pixel binning The full MDIS instrument includes the pivoting dual camera system as well as two redundant external Data Processing Units (DPUs). The dual camera assembly without the DPUs is simply referred to as 'MDIS'. Only one DPU may be active at a time, and due to thermal constraints only one camera will operate at a time; however, observations with the two cameras can be interleaved at 5-second intervals. A separate electronics assembly accommodates switching between the various modes of operating with the redundant DPUs. The pivot platform has a large range of motion (240 degrees) to allow the cameras to be 'tucked away' to protect the optics from contamination. The pivot motor drive-train provides precision rotation over the 90-degree operational range of motion about the spacecraft +Z axis. A spectral calibration target is mounted on the inside of the payload adapter ring. Early in the MESSENGER mission it was possible to tilt the spacecraft in order to provide solar illumination on the calibration target. The large range of motion of the pivot assembly enabled either camera to point at the target permitting an absolute inflight radiometric calibration and flat-field measurement. Optical Design ============== The WAC consists of a 4-element refractive telescope having a focal length of 78 mm and a collecting area of 48 mm**2. The detector located at the focal plane is an Atmel (Thomson) TH7888A frame-transfer CCD with a 1024x1024 format and 14-micron pitch detector elements that provide a 179- microrad pixel (instantaneous) field-of-view (IFOV). A 12-position filter wheel provides color imaging over the spectral range of the CCD detector. Eleven spectral filters spanning the range from 395 to 1040 nm are defined to cover wavelengths diagnostic of different potential surface materials. The twelfth position is a broadband filter for optical navigation. The filters are arranged on the filter wheel in such a way as to provide complementary bandpasses (e.g., for 3-color imaging, 5-color imaging, etc.) in nearby positions. The NAC is an off-axis reflective telescope with a 550-mm focal length and a collecting area of 462 mm**2. The NAC focal plane is identical to the WAC's, providing a 25-microrad IFOV. The NAC has a single medium-band filter (50 nm wide), centered at 750 nm to match to the corresponding WAC filter for monochrome imaging. One of several impacts of the thermal environment on calibration accuracy is the relative responsivity of the CCDs at wavelengths longer than about 700 nm. Response at longer wavelengths increases strongly with temperature. If data are acquired over a large temperature range, inaccuracies in correction for temperature-dependent response introduce systematic errors in spectral properties at 850-1000 nm that ultimately could lead to false mineralogic interpretations. To protect the MDIS CCDs from wide temperature swings, incoming thermal infrared radiation (IR) is rejected in the optics by heat-rejection filters on the first optic of each camera. In the WAC, this rejection is accomplished using a short-pass filter as the outer optic; for the NAC the bandpass filter has a specially designed heat-rejection coating on its first surface. WAC Design ========== Telescope Design. The WAC consists of a refractive telescope, dictated by the required wide FOV and short focal length. The design approach was to select the simplest lens design that gives acceptable image quality over the field; however, an important constraint on the design is the limited selection of glasses because of the radiation environment. The design requirements were achieved by starting from a simple Cooke triplet and splitting the central negative element. The resulting design, a Dogmar, gives good image quality although not over the full 395-1040 nm spectral range. The uncorrected axial chromatic aberration was reduced by varying the thickness of each filter. The WAC is focused at infinity with a 78-mm focal length to spread the 10.5 degree FOV across the 14.3-mm detector. The 14-micron square pixels of the CCD provide an instantaneous FOV (IFOV) of 179 microrad. The field curvature of the WAC produces a small focus variation across the field, but it has a negligible effect on the final modulation transfer function (MTF). Optical distortion has a value of 0.06 percent at the corner of the field, less than a pixel. The design is diffraction-limited over the entire field, and MTF degradation is dominated by the pixel size. Spectral Filter Design. A 12-position multispectral filter wheel provides color imaging over the spectral range of the CCD detector (395-1040 nm). Eleven spectral filters are defined to cover wavelengths diagnostic of common silicate minerals and glasses and have full-width half maximum (FWHM) bandwidths from 5-40 nm. A broadband clear filter is included for optical navigation imaging of stars. Each filter consists of two or three pieces of glass using a radiation- resistant substrate in combination with a long-pass filter. Two filters required an additional layer to achieve the desired passband. The variation in filter thickness used to remove residual chromatic aberration results in a small variation in the focal length of the camera between filters. The extreme filters give a focal length of about 78 mm at 480 nm and about 78.5 mm at 1020 nm respectively. This difference results in a variation in the image scale of 0.7 percent. By positioning the filter in front of the detector, the size of the filters is minimized. However, the incident angle of the beam on the filter varies with field angle, causing a shift in the spectral bandpass of the interference filters across the field. The effect of the incident angle of the converging rays on the filter is much more serious than the spatial variations across the surface of the filter. With a maximum angle of 7.6 deg. at the corner of the field, spectral shifts of about 3 nm are expected. NAC Design ========== The primary purpose of the NAC is for high-resolution imaging of Mercury. Because of the very bright optical signal from the planet, a large collecting area is not required for sensitivity. The reflective design has a long focal length that required folding the optical path in order to fit in the available volume. The monochromatic design has a single medium- band filter centered at 750 nm with a FWHM of 50 nm. The center wavelength was chosen to match a corresponding WAC filter to facilitate switching between cameras for global mapping. The NAC has a 1.5-degree FOV that is spread across the 14.3-mm detector, requiring a focal length of 550 mm. The NAC has a 25-mm aperture, resulting in an F/22 system. An off-axis Ritchey-Chretien design was selected over a Cassegrain in order to avoid the central obscuration of the secondary mirror. The ellipsoidal primary mirror and hyperboloidal secondary mirrors are gold-coated aluminum. In this design, the image plane is tilted at an angle of approximately 9 degrees to locate the optimal image quality. A bandpass filter is the first optical component of the assembly and defines the spectral range of the instrument. A specially designed interference coating serves as a heat-rejection filter to minimize admittance of IR radiation from Mercury that would heat the CCD during imaging and increase background levels. Due to mass limitations the aperture is less than required for diffraction limited performance, so the Airy disk is approximately 2 pixels across. Low distortion of the image was an important design specification for the NAC. At the edge of the FOV this amounts to 1.28 pixels. The all-aluminum assembly of the NAC makes it insensitive to thermal distortions over the operational temperature range of the instrument. Filters Bandpasses ================== The WAC camera utilizes a twelve position filter wheel with band passes between 415 and 1020 nm. The NAC is a broadband imager with a bandpass centered near 750 nm. Filter Filter Center Bandpass Number letter wavelength width in file (nm) (nm) name 1 A 698.8 5.3 2 B 700 600.0 3 C 479.9 10.1 4 D 558.9 5.8 5 E 628.8 5.5 6 F 433.2 18.1 7 G 748.7 5.1 8 H 947.0 6.2 9 I 996.2 14.3 10 J 898.8 5.1 11 K 1012.6 33.3 0 12 L 828.4 5.2 NAC M 747.7 52.6 Pivot Mechanism =============== The MDIS pivot platform is controlled by a stepping motor whose phases are controlled directly by the DPU software to move the platform. The phase pattern can be adjusted by software to move the platform forwards or backwards. The pivot platform's range of motion is mechanically constrained by hard stops. The range of motion is further constrained by soft stops applied by the software. The total range of motion of MDIS is about 240 degrees, limited by hard mechanical stops in the pivot motor. The hard stops are fixed at -185 degrees and 55 degrees from the spacecraft +z axis. The pivot motor drive-train provides precision rotation over the 90 degrees operational range of motion 40 degrees sunward and 50 degrees antisunward of the spacecraft +z axis. The MDIS pivot actuator is capable of accurately stepping in intervals of 0.01 degrees (about 150 microrad) per step. Pointing knowledge is determined by first 'homing' the instrument, which is accomplished by driving the actuator into one of the mechanical hard stops for a period of time sufficient to ensure the orientation of the instrument if it had been previously stopped at the opposite extreme of travel. Once the location of the pivot actuator is known, the flight software retains this knowledge and subsequent pointing commands are achieved by counting pulses (steps) to the motor. Thermo-mechanical Design ======================== The MESSENGER spacecraft is protected from direct solar illumination by the spacecraft sunshade. The thermal environment of the instruments is largely benign with the exception of short intervals during the 12-hour orbital period when Mercury's illuminated surface subtends a large solid angle. The passive thermal design of MDIS buffers temperature changes to prevent the instrument from exceeding the CCD's high operational temperature limit (-10C to limit the noise contribution from dark current) except for occasional short periods. Survival heaters ensure that cold operational limits are not violated. The DPU is directly coupled to the spacecraft deck and blanketed with multi-layer insulation (MLI). The -40 C to +50 C operating temperature of the DPU follows the deck temperature. The temperature of the imager interface board and the low-voltage power supply board are measured internal to the DPU and reported in the instrument housekeeping data. The MDIS temperature sensors are mounted at key locations on the instrument and are reported in image headers. The CCD temperature is the most important temperature for calibration. Thermal Design of the MDIS Platform Assembly. Each camera has its own thermal control system, and each CCD is mounted to a heat sink that bolts onto a bracket that is thermally isolated from the focal plane unit (FPU). Titanium spacers minimize heat conduction from the FPU to the CCD while maintaining focus over the operating temperatures of the instrument. A thermal link made of made of multiple sheets of thin aluminum foil ties the CCD to a phase-change material (wax pack). A thermal strap, made of copper braid secured to the top of the wax pack using a conductive epoxy, thermally ties the wax pack to the evaporator portion of a stainless steel, butane-charged diode heat pipe. Each camera has its own wax pack and heat pipe assembly. However, the heat pipes for both cameras are connected to a common radiator assembly made of three beryllium plates mounted to the instrument with titanium spokes. The heat pipes, fabricated by Swales Aerospace, are made from 9.5-mm- diameter stainless steel tubing filled with about 5 g of butane. During normal operation, heat flowing from the CCD and through the wax pack is absorbed at the evaporator. Liquid butane is evaporated and the gaseous butane flows down the center of the tube toward the radiator to be condensed on the cold walls of a condenser. An internal stainless steel wire mesh acts as a wick to transport the liquid from the condenser back to the evaporator, continuing the fluid loop. A separate pipe, the liquid trap, is attached to the evaporator section that allows the heat pipe to act as a diode, i.e. allowing heat to flow only in one direction. A small tube connects the liquid trap to the evaporator. The diode action occurs when the condenser/radiators becomes warmer than the evaporator and the butane liquid in the condenser vaporizes. The now gaseous butane flows down the pipe and condenses onto the walls of the relatively cool (-10C) evaporator and liquid trap. Since there is no wick in the small tube between the liquid trap and the evaporator, the liquid cannot flow out of the trap, back to the evaporator, and up to the condenser for re-evaporation. As the radiator plates continue to heat up, more liquid is vaporized and flows down to the liquid trap area. This process continues until most of the liquid is located in the liquid trap and no more liquid can flow back up the pipe to the condenser. In this state, the heat pipe is effectively shut off and there is no longer any heat transfer between the evaporator and condenser. Once the radiators cool back down and the condenser again drops below the evaporator temperature, the heat pipe will start back up and conduct heat like a regular heat pipe. On orbit at Mercury, when the spacecraft is away from the hot planet, the MDIS radiator plates do not see the planet and are only exposed to deep space. In this condition, the radiator plates become very cold (-77C) and the diode heat pipes operate as normal heat pipes. The gas internal to the pipe moves freely along the length of the pipe, transporting heat from the wax pack out to space. The heat from the CCD flows from the CCD through the thermal link, through the wax pack and thermal strap, to the evaporator of the heat-pipe. As the liquid in the pipe evaporates, heat flows along the heat pipe to the condensers and finally to the radiator panels. When the paraffin in the wax pack is all frozen, the wax pack, thermal link, and CCD continue to cool below the freezing point of the paraffin. As the spacecraft starts to approach the planet, the radiator plates begin to absorb heat from the hot planet. When the temperature of the condenser rises above the temperature of the evaporator, most of the heat-conducting gas in the pipe condenses in the liquid trap, effectively shutting off the heat pipe. Heat that is still flowing from the CCD will now be absorbed into the wax. The paraffin warms up to its melting point and remains at that temperature until all 240 g of paraffin has melted. The amount of wax was selected so that all the wax would not completely melt during the hours surrounding periapsis. The melting wax clamps the temperature of the CCD at warm operating temperature limit (-10C). As the spacecraft moves farther away from the planet, the MDIS radiator and condenser temperatures fall below the evaporator temperature and allow the heat pipe to operate normally again. With the condenser temperature below the evaporator temperature, heat flows from wax pack to the radiators and begins refreezing the paraffin. The partially melted wax pack remains at its melting (freezing) point until all the paraffin solidifies. At that point, the wax pack temperature continues to drop down to -45C where heaters and thermostats control the minimum temperature. Other than the CCD-to-heat pipe heat path, the majority of the camera components are thermally connected to the main platform. Heaters on the platform prevent the camera optics and FPU electronics from dropping below -35C. There are no radiators associated with the platform. The cameras and platform are isolated from the environment by MLI blankets and do not overheat due to the relatively small amount of heat dissipated in the electronics. Detectors ========= The CCD used in both the NAC and WAC is an Atmel (formerly Thomson-CSF) TH7888A with 14-micron square pixels and antiblooming control, that uses frame transfer to obtain electronic exposure control. The active image forms an array 1024x1024 pixels (14.3x14.3 mm) in size in which optical energy is accumulated. At the end of the exposure period, the accumulated charge in each pixel is transferred in about 3.4 ms to the masked-off 1024x1024 memory zone. A tantalum radiation shield not only protects the CCD from ionizing radiation but also blocks illumination to the memory zone. The combined image and memory zones' 1024x2048 pixel array has two on-chip output amplifiers, though only one amplifier is used in the MDIS design. Manual and autoexposure control from 1 ms to 9989 ms permit imaging over a broad range of intensities. The CCD provides 4 specially shielded columns to provide an accurate measure of the dark level in a full resolution image (1024x1024). In the case of 2x2 binning, the corresponding columns are two wide but shifted out of the shielded area of the CCD. The antiblooming feature of the CCD causes a scene-dependent nonlinearity at high DN levels. Small sources do not typically exceed 3500 DN, but on larger sources where antiblooming is overwhelmed, higher DN values can occur. Electronics =========== The main electronics systems of MDIS include the DPUs, the DPU Interface Switching Electronics (DISE box), and the FPU camera electronics. Focal Plane Electronics. The detector electronics for both the WAC and NAC are identical. Each CCD is bonded to a camera-specific mounting bracket (heat-sink) prior to assembly into the FPU electronics. The NAC heat-sink is tilted 9 degrees to match the optimal orientation of the NAC focal plane, whereas the WAC heat-sink has no tilt. Due to thermal, power, and operational constraints, only one camera operates at a time. The frame rate of each FPU is fixed at 1 Hz; however, the frame rate to the spacecraft is not fixed but cannot exceed 1 Hz. The focal plane electronics are reduced in size to save mass. A type of construction of the printed circuit boards known as rigid-flex enabled the electronics to be folded into a small space. The clock drivers use an integrated circuit (IC) designed to drive the gates of power metal oxide semiconductor field effect transistors (MOSFETs) and are almost ideal to drive the capacitive gates of a CCD. The TH7888A has a relatively high internal gain of 6 microV/e. This value is large enough to drive the correlated double sampler analog-to-digital converter (ADC) integrated circuit directly while meeting the system noise requirements. The output is a 12-bit digital word for each CCD pixel value. A Field-Programmable Gate Array (FPGA) is used to provide clocks for the CCD and ADC derived from an input clock from the DPU. The DPU also provides a coded exposure time signal for the camera, as the autoexposure algorithm is in the DPU. The FPGA also formats the data for transfer back to the DPU. The interface between cameras and DPU uses low-voltage differential signaling for low noise and to tolerate differences in grounding between the units. DISE. The MDIS instrument contains six major electrical subsystems: two FPUs, a filter wheel motor, a filter wheel motor resolver, a platform pivot motor, and a platform pivot motor resolver. The pivot motor contains redundant windings with one set going to each DPU. The remaining systems must be controllable by either of the two redundant DPUs. Selection between these units is provided by the DISE box. The DISE box switches between the DPU and MDIS electronics while minimizing the connections through the rotary twist-capsule feedthrough. There are two switching functions: one selects which DPU is master and the other selects which camera will be active. After the DPU asserts itself as master, it then applies power to the camera of interest. Each DPU provides separate power lines for each camera. The DISE board uses these power connections to power its internal circuitry that selects the correct camera as well as powering the camera of interest. This selection is done in a master/slave configuration with the NAC having priority. If power is supplied to both cameras at the same time, signals from the NAC will be transmitted. This arrangement ensures that the master DPU communicates with the intended camera. In addition to delivering each camera with the 15 V provided by the master DPU, the DISE box linearly regulates a 6-V power supply. This regulation is done in the DISE box to limit power dissipation and reduce mass in the cameras. To ensure good thermal conduction of these components so that they can safely dissipate their heat, they are mounted directly to the lower corners of the DISE box frame. The DISE box switches control lines using the low-voltage differential signaling (LVDS) protocol to and from each camera. Each camera has a single LVDS interface, and each DPU has a single LVDS interface. Switching between them is done with cold sparing LVDS receivers and cold sparing complementary metal oxide semiconductor (CMOS) multiplexers. Data Processing Unit. The DPU electronics provides two distinct services: it acts as a communications router and interface between all the instruments and the spacecraft, and it provides complete support for the MDIS camera electronics. Two separate sets of electronics boards are physically stacked and packaged together. The Event Processing Unit (EPU) board provides data processing capability and creates the telemetry data packets passed (through the DPU) to the spacecraft. Its design is based on the Intersil (formerly Harris) RTX2010 16-bit processor. In addition to the RTX2010, the EPU board contains 64 kB of fuse-link programmable read-only memory (PROM) for boot code, 256 kB of electrically erasable PROM (EEPROM), and 256 kB of SRAM. An Actel RT54SX32S FPGA provides all the logic necessary to interconnect the processor with the stacking connector data bus. The EPU also receives (via the DPU) a once per second timing pulse from the spacecraft. This synchronization pulse is a key part of the overall timekeeping design to maintain <1-ms end-to-end timing error between the mission operations center and the payload. The Low Voltage Power Supply (LVPS) board converts spacecraft primary power, ranging from 20 V to 36 V, to isolated secondary power. The LVPS board utilizes an in-rush limiter, a VPT electromagnetic interference (EMI) 461 filter, and two VPT hybrid dual output converters (one for +/-5 V and the other for +/-12 V). These converters provide better than 1 percent load and line regulation for nominal operation. Two radiation- hardened opto-field effect transistor (FET) relays were used to switch up to 1 A of primary power to off-board loads. An analog monitoring circuit, comprised of a 14-bit ADC, instrumentation amplifiers, a 16-1 multiplexer, and an Actel FPGA provided 16 input analog channels, which were used to monitor four voltages and six currents on the board, and five analog voltages from off the board; the remaining channel measured board temperature. Communication between each DPU and the instruments (other than MDIS) takes place through a Universal Asynchronous Receiver/Transmitter (UART) hub board with eight 38.4-kbaud, RS-422, full duplex UART channels. Each channel has a separate first circuit interface for maximum fault tolerance. Independent SRAM-based receive buffers indexed by FPGA-based registers are used to offload byte-by-byte operations from the DPU software. Communications between each DPU and the spacecraft is accomplished through MIL-STD-1553 buses. A 1553 Interface Board has a design is based on the UTMC SUMMIT UT69151DXE protocol/transceiver chip and also includes an Actel RT14100A FPGA and 64 KB of SRAM. Each DPU is a remote terminal on the spacecraft 1553 bus. All payload commands and telemetry, except MDIS image data, are communicated via the 1553 bus. All image acquisition control and hardware image processing occurs in the Imager Interface Board. It utilizes two Actel RT54SX72S FPGAs, 2 MB of SRAM, and a number of LVDS receiver and transmitter circuits to communicate both with the FPUs and the spacecraft high-speed Integrated Electronics Moduloe (IEM) interface card. The Imager Interface Board contains two FPGA designs: an imager- interface FPGA and an RTX-bus FPGA. Image data are continuously received from the active FPU and buffered in SRAM. If the DPU receives a command to record an image, the image data are transferred to either of the two redundant IEMs on the spacecraft. The main function of the imager- interface FPGA is to read images from the FPU and send them to the IEM. The image data read from the focal plane are received from a serial LVDS interface at 16 MHz. A full 1024x1024 image requires approximately 850 ms to be received in the DPU. The data are sent at 4 MHz to the IEM, limiting the image frame rate. 512x512 on-chip binned images may be sent to the IEM at 1 Hz; however, full-frame images can be sent only at 0.25 Hz. This interface limitation to the IEM requires the imager-interface FPGA to buffer the images in SRAM. The main functions of the RTX-bus FPGA are to store a 1024-bit image header and the FPU command words generated by software running on the RTX processor in the DPU. This FPGA is also capable of generating a 32-bin histogram created from the pixel values produced by the FPU. This histogram is made available to the RTX processor and is integral to the MDIS autoexposure algorithm. The MDIS Imager Power Board provides switched +15 V to the NAC and WAC electronics. A separate MOSFET-switched output is used for each NAC and WAC FPU electronics. Each of the pivot and filter wheel actuators has a dedicated Motor Controller Board. Although the characteristics of the two actuators are different, the board designs are effectively identical. Each board utilizes an EMI filter, an Actel A1020 FPGA, and a bipolar UDS2998 motor- driver to generate the appropriately phased motor drive waveforms. The angular position of each actuator is measured via an AD2S80A motor resolver readout hybrid. Software ======== The APL-developed MESSENGER instruments share common software, an instrument domain library. The library was created in 1994 to serve the NEAR instruments. It is currently used in MESSENGER as well as the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on Mars Reconnaissance Orbiter and the LOng Range Reconnaissance Imager (LORRI) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instruments on New Horizons. The common software runs as a layer below the application-specific software. The first layer provides standard services, for example, command handling and telemetry packet queuing. Another layer below the service layer is a host abstraction layer. This layer abstracts the input/output (I/O) interfaces used by the instrument to communicate with the spacecraft. The common software includes a boot program. Its most frequent use is to boot application code by copying it from EEPROM to RAM and then start the application. However, the boot program also has commands for loading memory from the ground, dumping memory back to the ground, and copying memory to and from EEPROM. These features can be used to upload new instrument programs. The boot program is stored in non- volatile ROM. In the event that an instrument's program in EEPROM gets corrupted, the boot program will allow the instrument software to be reloaded. The common command software can store and run sequences of commands, called macros. Macros can loop and call other macros. Up to 64 macros can be executing at any time. The common software's telemetry service provides formatting and queuing of telemetry packets. The software gathers the packet's data and constructs a Consultative Committee for Space Data Systems (CCSDS) header with the appropriate flags, sequence number, and other information. The packet is queued for transmission to the spacecraft. The common software also includes status/housekeeping packet generation, time management, voltage and current monitoring, and I2C bus management services. MDIS-specific software controls the imager hardware, exposure time, and mechanisms. Commands are accepted from the spacecraft and forwarded to the appropriate instrument. Telemetry is collected from the instrument and forwarded to the spacecraft. Imager Control. Only one of the MDIS cameras, either the NAC or WAC, can be used at any given time. The MDIS software selects the camera to use based on user command. Images are collected in three stages: expose the image in the FPU, read the image from the FPU to the DPU, and finally transmit it from the DPU to the spacecraft. Different processing steps are applied to the image during each of these stages. The steps include exposure, optional 2x2 binning, optional lossy 12-to-8-bit compression, and optional lossless FAST compression. Exposure Control. The exposure time of images can be set manually by command or automatically by the software. In manual mode, a full 1 ms to 9989 ms range of exposure times is available. In automatic mode, the exposure time of the next image is computed by the software but limited to between 1 ms and 989 ms. This computation has two distinct steps. The first step computes a new exposure time based on the brightness of a previous image. The second step anticipates filter wheel motions and adjusts the computed exposure time accordingly. During the read stage of the image pipeline, the hardware generates a histogram of the image. The histogram is analyzed by the software to determine if the image is overexposed or underexposed. First, the histogram is scaled by a factor of four if it comes from a 2x2 binned image. If the brightest histogram value exceeds a saturation threshold (typically 3500 DN), the image is considered overexposed and the exposure time is scaled back. Otherwise the image is considered underexposed. Histogram values are accumulated starting from the brightest bin down towards the dimmest bin, until a commandable, allowed number of pixels exceeding a target brightness is reached. The brightness value that causes the sum to exceed the allowed value is the actual image brightness. The exposure time is scaled by the ratio of the commanded target brightness minus a background brightness (dark current) to the actual brightness minus the background brightness. The algorithm uses uploadable parameters for the saturation threshold, overexposure fallback, background brightness, target brightness, and the number of pixels allowed to exceed the target brightness. Next, the algorithm adjusts the exposure time further if the imager, binning mode or filter selected for the next exposure does not match what was used in the test exposure. This is done using an uploadable 'filter transmissivity table' containing relatively sensitivities of the 12 WAC filters and the NAC in binned and unbinned modes (26 values). The exposure time is scaled by the ratio of the transmissivity of the old setup to the transmissivity of the new setup. Finally, the computed exposure time is forced to fall within an uploadable range but is always less than 989 ms; this latter step prevents too-long exposures to limit image motion smear. Mechanism Control. The MDIS pivot platform and filter wheel are controlled by stepper motors. The pivot can move about 1.1 deg/s. Similarly, the filter wheel can move 75 deg/s. Given twelve filters, an adjacent filter can be reached in 0.4 s and any filter within 2.4 s. Image Compression ================= The MESSENGER mission requires compression to meet its science objectives within the available downlink. Several successive options are available at the instrument level using the DPU, and using the spacecraft main processor (MP). At the instrument level, (1) 2x2 binning is available on-chip to reduce the 1024x1024 images to 512x512 format. (2) 12-bit data number (DN) levels can be converted to 8 bits using any of eight look-up tables. (3) Data can be compressed losslessly using the Fast algorithm. The strategy for image compression at the instrument level is to acquire all monochrome data in 8-bit mode, all color data in 12-bit mode, and to compress all data losslessly to conserve recorder space. After data are written to the recorder, they can be uncompressed and recompressed by the MP more aggressively using any of several options: (4) Additional pixel-binning can be performed, including options for 2x2 or 4x4 compression of the input image. In practice, no more than 4x4 total compression (including on-chip and by the MP) is applied to preserved the dark columns. (5) Up to 5 subframes of the image can be saved, or the whole image can be retained. (6) For the special case of optical navigation images, there is a 'jailbar' option that saves selected lines of an image at a fixed interval for optical navigation images of Mercury during flyby approaches. (7) Compression using an integer wavelet transform can be applied, except when jailbars are used. The strategy for MP compression is that key inflight calibrations will be lossless compressed by specifying a compression ratio of 1:1, whereas all science data will be lossy compressed. Compression ratios are typically 8:1 for monochrome data and 4:1 for orbital color data. Color imaging but not monochrome imaging may be further pixel-binned. Ground Calibration Facility =========================== All calibration tests were performed in the Optical Calibration Facility (OCF) at APL. The OCF consists of large, linked vacuum chambers with a host of support equipment. The largest of these chambers, the instrument chamber, has an internal diameter of 1.3 m and a length of 2 m and permits mounting the instrument on a three-axis motion stage. The motion stage can rotate the instrument in azimuth and elevation through a range limited only by the instrument harness and mounting hardware. Translation is possible over a limited range, but it is sufficient to center the instrument in the collimated beam of the chamber. The interior and ends of the instrument vacuum chamber are surrounded by cold walls, within which an external refrigerator circulates a cooling fluid to reduce the internal temperature to approximately -40C. Most of the calibrations were performed at approximately -30C. However, additional thermal configurations over the range -34C to +25C were also performed, to bracket the behavior of the instrument over the range of expected flight conditions. Inside the instrument chamber, there are two sources of calibrated illumination. In one configuration the instrument views a 20-inch diameter integrating sphere through a quartz window. Dark cloth was draped around the window and sphere opening in order to block unwanted external light from entering the instrument optical path. Sphere spectral radiance was calibrated by the manufacturer just prior to the start of instrument calibration, for each configuration of the sphere's four bulbs: two 45-W lamps and two 150-W lamps. One photometer was coupled through a fiber optic to monitor the internal sphere brightness during all calibration runs. Two additional photometers were mounted on the camera and allowed additional monitoring of incident irradiance. Rotating the azimuthal stage of the OCF 180 degrees enabled the instrument to look along a beam tube into an off-axis parabolic collimating mirror, which has a focal length of 1.43 m, operates at F/7, and is focused on a grating monochromator. The monochromator may be illuminated by an incandescent lamp with quartz optics. At the input slit, a set of neutral density filters may be used to attenuate the light by known amounts. At the exit slit, a set of long-pass filters is used to remove higher orders (i.e., shorter wavelengths) from the grating. The wavelength can be changed manually or can be set to scan under computer control. The grating may be positioned to zeroth order, which allows the full incandescent spectrum to be passed. A MgF2 window partitions the monochromator from the unit under test, allowing the monochromator to be removed without breaking vacuum so that other sources may be placed at the focus of the collimator. These sources include a point source (pinhole), pinhole array, and test samples illuminated by an incandescent source and viewed off a silver fold mirror. Four networked computers were used to acquire and monitor the calibration data. Calibration Equation ==================== Laboratory and flight measurements were used to derive values for the terms of the calibration equation for both the WAC and NAC. Both instruments measure relative light intensity in engineering units referred to as DNs. The raw engineering units are converted to the physical units of radiance, following the calibration equation: L(x,y,f,T,t,b) = Lin[DN(x,y,f,T,t,b,MET) - Dk(x,y,T,t,b,MET) - Sm(x,y,t,b)] / {Flat(x,y,f,b) * t * [Resp(f,b,T)/Correct(f,MET)]} where: L(x,y,f,T,t,b) is radiance in units of W / (m**-2 microns**-1 sr**-1), measured by the pixel in column x, row y, through filter f, at CCD temperature T and exposure time t, for binning mode b, DN(x,y,f,T,t,b,MET) is the raw DN measured by the pixel in column x, row y, through filter f, at CCD temperature T and exposure time t, for binning mode b, and Mission Elapsed Time (MET), Dk(x,y,T,t,b,MET) is the dark level in a given pixel, derived either from the dark strip or estimated from exposure time and CCD temperature, Sm(x,y,t,b) is the scene-dependent frame transfer smear for the pixel, Lin is a function that corrects small nonlinearity of detector response, Flat(x,y,f,b) is the non-uniformity or 'flat-field' correction at this pixel location, Resp(f,b,T) is the responsivity, relating dark-, flat-, and smear-corrected DN per unit exposure time to radiance, Correct(f,MET) is a time-variable correction to responsivity describing a sudden decrease in transmission of the WAC optics on 24 May 2011 and subsequent recovery to normal values, interpreted as due to contamination associated with MESSENGER's first periapse season over Mercury's hot pole and the subsequent bake-off of the contaminant, and t is the exposure time in milliseconds. The above equation assumes that data are in the native 12-bit format in which they were read off the CCD, and that onboard application of 12-to-8 bit lookup tables (LUTs) has been inverted. This correction is done step-wise using the calibration tables and images in this directory as follows. Inversion of 12 to 8 bit Compression ==================================== The equation above assumes that DN levels are in their original 12-bit form. For any image that has compressed to 8 bits, the inversion is performed using the inverse look-up tables provided in the CALIB directory. Dark Level ========== The measured signal from the CCD, in the absence of incident photons, is the sum of three major components: (1) dark current from thermal electrons, (2) an electronic offset, or bias, of about 240 DN intentionally added to the readout to prevent occurrence of negative values that would appear as zero DN words, and (3) noise picked up by instrument electronics. There has been no evidence of picked-up noise. In the following discussion, the sum of dark current and bias is treated together as 'dark level.' The response of pixel-dependent dark level to temperature and exposure time was characterized separately for the NAC and WAC without on-chip binning ('full-frame') and with on-chip binning, over the CCD temperature range 25C to -40C. In the NAC full-frame data, at short exposure times and low temperatures, the dark level is dominated by the electronic bias and the image mean decreases with increasing temperature. This relationship originates in the amplifier in the read-out electronics and has a temperature-dependent offset. After an image has been exposed, large current pulses are sent from the power supply to the clocking signals controlling the movement of charge in the CCD. These high-current clocking pulses increase the overall signal levels for a very short time, as observed in the measured DNs. At longer exposure times (>200 ms) and higher CCD temperatures (> -30C) the effects of dark current, which increases exponentially with temperature, dominate. In binned images, because on-chip binning adds the outputs of four pixels prior to quantization, the impact of the dark current is larger and the onset of increasing dark level with temperature and exposure time occurs at lower temperature and shorter exposure time. WAC dark levels are dominated by the increase in dark current with higher temperature and longer exposure time. Detector Temperature Calibration: CCD temperature is measured by a temperature sensor bonded to the CCD heat-sink mounting plate on each camera. This temperature sensor, whose value is reported in instrument housekeeping, was calibrated during dark level testing, where it was possible to install multiple thermocouples throughout the MDIS instrument. In particular, a thermocouple located in close proximity to the thermal link on the wax pack of each FPU was used as the best measurement of the actual CCD temperature. A linear regression was used to fit the measured response of the CCD temperature sensor with the calibrated thermocouple response. The other instrument temperature sensors, located on each FPU housing and the WAC filter wheel and NAC telescope, were calibrated in a similar fashion. The final expressions to convert the digital raw output to temperature are of the form T(deg c) = A * raw_counts + B Camera Temperature Name A B WAC CCD temperature MESS:CCD_TEMP 0.2718 -318.455 WAC FPU temperature MESS:CAM_T1 0.5022 -262.258 WAC filter wheel temp. MESS:CAM_T2 0.553 -292.760 NAC CCD temperature MESS:CCD_TEMP 0.2737 -323.367 NAC FPU temperature MESS:CAM_T1 0.5130 -268.844 NAC Telescope temp. MESS:CAM_T2 0.4861 -269.718 Dark Current Model: For the purpose of modeling dark current, the WAC and NAC were treated as four different sensors: NAC full-frame, NAC binned, WAC full-frame, and WAC binned. Two dark models were derived from the OCF calibrations for each of the four modeled sensors: (1) a forward model of the dark current as a function of column number, row number, exposure time, and raw CCD temperature (DN), and (2) a backward model that uses the row-dependent dark levels in the dark strips and extrapolates them across the FOV using the same variables as the forward model. For both the forward and backward models, dark level is treated as a linear function of column number, row number, and exposure time. The exponential temperature dependence was approximated with a third-order polynomial for ease of calculation. While it is known that the CCD temperature response will change with time due to exposure to radiation in space, this specific time dependence will be determined in flight over the lifetime of the mission. The forward is more accurate and is described below. Consider the first of four variables to the dark model: column position x, row position y, exposure time t, and temperature T. For the column- dependence at a particular row, exposure time, and temperature, dark level is modeled linearly where an offset alpha(y, t, T) is the value at column x=0, and a slope beta(y, t, T) is the increase per column (per increment of x). Then, to find the dark level at a given column, the model for this row, exposure time, and temperature is a linear function: Dk(x,y,t,T) = alpha(y,t,T) + beta(y,t,T) * x However, the column-dependence will vary with row, so that in the second level of the model, the offset alpha and slope beta are themselves a function of row position y. Thus, linear expansions are: alpha(y,t,T) = A(t,T) + B(t,T) * y beta(y,t,T) = M(t,T) + N(t,T) * y Note that at this level of expansion of the dark model, the row- dependences of column-dependences are themselves dependent on exposure time and temperature. A third level of linear regression can be added to the model, such that A(t,T), B(t,T), M(t,T), and N(t,T) can each be expressed as its own linear function of exposure time t, using a temperature-dependent offset C(T), O(T), E(T), or Q(T) respectively, and a temperature-dependent slope D(T), P(T), F(T), or S(T) respectively. The expanded dark model at this level becomes: Dk(x,y,t,T) = C(T) + D(T) + [E(T) + F(T) * t] * y + {O(T) + P(T) * t + [Q(T) + S(T) * t] * y} * x At the fourth and final level of expansion, C(T), D(T), E(T), F(T), O(T), P(T), Q(T), S(T) are all third-order functions of temperature, for example: C(T) = H0 + H1 * T + H2 * T**2 + H3 * T**3 In all cases x or y is in the range 0-1023 for a not-binned image or 0-511 for a binned image, t is in units of milliseconds, and T is in UNCALIBRATED raw counts of CCD temperature. The forward dark current model was derived by first finding a linear fit of dark level as a function of column number for each row in the dark calibration data. Second, the bias and dark DN accumulation rate per column increment were fitted linearly as functions of row. A, B, M, and N were fitted as functions of exposure time, for data binned into fifteen difference CCD temperature bins (each bin 10 DN wide) covering the range 1000-1150 DN (-40.8C to -6.8C). Finally, the linear fit coefficients as functions of exposure time at different temperatures were fitted as a function of temperature with a third-order polynomial. The coefficients of these expansions are provided in the CALIB directory. Frame Transfer Smear Correction =============================== An image is exposed for a nominal integration time and is then transferred to the memory zone of the CCD, from which the analog signal is digitized line by line. Accumulation of signal continues during the finite duration of frame transfer, inducing a streak or frame-transfer smear in the wake of an illuminated object in the field of view, parallel to the direction of frame transfer. Quantitatively, the smear correction is: Sm(x,y,t,b) = SUMM(1,y-1) { t2/t * [DN(x,y,t,b) - Dk(x,y,T,t,b) - Sm(x,y,t,b)] / Flat(x,y,b,f)} where Sm(x,y,t,b) is the smear in column x and row y at exposure time t in binning mode b, Dk(x,y,t,b) is the dark level in column x and row y at exposure time t and temperature T in binning mode b, Flat(x,y,b,f) is the flat-field correction in column x and row y in binning mode b and filter f, t is exposure time in ms, and t2 is the time for frame transfer (about 3.4 ms) divided by the number of lines in the image in the direction of frame transfer, i.e., 1024 for full-frame images or 512 for binned images. Response Linearity Correction ============================= Calibration of the signal accumulation rate per unit time per unit radiance at the sensor was conducted in the OCF for the WAC and the NAC over a broad range of exposure times and source light intensities using a white integrating sphere. The exposure times to saturation were determined empirically in initial set-up. Using a variety of temperatures (-34C to 25C) and source intensities, images were acquired after varying the exposure times until saturation occurred. Source intensities were stepped in a fixed decreasing-light pattern through the eight levels that could be achieved with the integrating sphere's two 150-W and two 45-W bulbs. The bulbs were allowed about 20 minutes of warm-up to asymptotically approaching constant light output. The WAC calibration procedure began with all lamps on then subsequently turned lamps off in sequence. The exposure sequence was repeated for each filter for the WAC, but only a single exposure sequence was required for the single filter of the NAC. The relationship between the raw DN output from the camera per unit exposure time and radiance, as a function of variation in exposure time or radiance is referred to as response linearity. Using dark-corrected, desmeared DN values taken from the center quarter of the images, response linearity was measured with both the NAC and WAC binned and full-frame. Linearity was first examined separately for (a) linearity with respect to exposure time at individual light levels, and (b) linearity with respect to radiance at individual exposure times. The results are not significantly different, so all data were merged to examine linearity with respect to measured photons (the product of radiance and exposure time). At corrected DN levels >1000, the CCDs are linear to within the error bars. However, there is a decrease in responsivity at lower DNs, by up to 2% at the lowest DN levels. To restore or 'linearize' WAC image data, the following corrections should be applied after correction of dark current, bias, and smear. For DN_dark > 1 DN_lin = DN_dark_smear/[0.008760 * Ln(DN_dark_smear) + 0.936321] For DN_dark <= 1 DN_lin = DN_dark_smear/0.936321 To restore or 'linearize' NAC image data, the following procedure should be applied after correction of dark current, bias, and smear. For DN_dark > 1 DN_lin = DN_dark_smear/[0.011844 * Ln(DN_dark_smear) + 0.912031] For DN_dark <= 1 DN_lin = DN_dark_smear/0.912031 where DN_dark is dark-corrected input DN DN_dark_smear is the input dark- and smear-corrected DN. DN_lin is linearized dark- and smear-corrected DN. Flat-field Correction ========================== The flat field correction removes pixel to pixel differences in detector responsivity, so that the responsivity coefficients can be expressed as scalars for each filter. The flat field is derived using a linearized, dark-corrected, desmeared image of a field- and aperture-filling, uniform source and normalizing the mean value to unity over the center quarter of the images. Two factors make the ground-derived flat-field less than ideal. First, MDIS's thermal blanketing generates reflections so that in the calibration chamber, the illuminated source created glint off the blanketing that reflected off the chamber window, adding spatially non-uniform stray light to the measurements. Second, eliminating the backscattering off the chamber window required acquisition of flat-fields at ambient (room- temperature) conditions at which residuals from the dark current correction introduce artifacts. Ultimately the latter approach was chosen for ground derivations. The flat-fields have been rederived inflight using images of a part of Venus near the sub-solar point, with a photometric correction applied to remove effects of variation of solar incidence angle across the scene. Flat-fields for different filters and binning states are given in the CALIB directory. Responsivity ============ Responsivity is linearized, dark-corrected, desmeared DN per unit time per unit radiance. Responsivity through each filter in each binning state (except WAC filter 2, which is too sensitive to be characterized with this experimental setup) was determined onground using the integrating-sphere, whose radiances was calibrated by Labsphere using each of the four lamps separately, over a wavelength range of 350-1100 nm in 5-nm steps. To determine radiances through each of the WAC and NAC filters, the band passes described below were convolved with the sphere's spectral radiance. For data taken at room temperatures, the chamber door was open, but for lower-temperature measurements the sphere was viewed through the quartz window in the OCF chamber door. Therefore for the low-temperature data, an additional correction was applied to the sphere radiance for each filter to account for the window. This correction was derived from room- temperature data, as the ratio of corrected DNs per unit time with the door closed to that with the door open, for a single configuration of sphere bulbs. It is expected the temperature-dependence of responsivity over the temperature range used will be approximately quadratic. The best correction available from the combination of flight and ground data is quadratic in form. The application of the linear correction is: Resp(f,T,b) = R(f,t=-30.3C,b) * [correction_offset(f,b) + T(CCD) * correction_coef1(f,b) + T(CCD)^2 * correction_coef2(f,b)] where Resp(f,T,b) is responsivity in filter f at CCD temperature T in binning state b, T(CCD) is raw CCD temperature in units of DNs, R(f,t=-30.3C,b) is responsivity in filter f in binning state b at CCD temperature of 1060 DN (-30.3C), correction_offset(f,b) is camera- and filter-dependent temperature correction offset for filter f and binning state b, correction_coef1(f,b) is the camera- and filter-dependent temperature correction first-order coefficient for filter f and binning state b. correction_coef2(f,b) is the camera- and filter-dependent temperature correction second-order coefficient for filter f and binning state b. To apply responsivity to obtain radiance L, the expression is L(f) = DN_flat(f) / (t * Resp(f,T,b) * Correct(f, MET)) where L is radiance in units of W / (m**2 microns**1 sr**1), DN_flat is dark-, smear-, linearity-, and flat field-corrected DN, t is the exposure time in milliseconds, Resp(f,T,b) is the responsivity in filter f at CCD temperature T and binning state b, and Correct(f, MET) is a temporal correction to responsivity for filter f at time MET. Values for Resp(f,T,b) and Correct(f, MET) for different filters and binning states are given in the CALIB directory. Filter Bandpasses: The NAC uses a single filter, while the WAC views the scene through a filter wheel outfitted with 12 filters of varying widths. The WAC filters are labeled by their position counter-clockwise around the wheel as viewed from the CCD. Filter 2 is a clear fused silica filter spanning the entire passband of the CCD (395-1040 nm), and filters 1 and 3-12 are narrowband filters. Using the OCF monochromator, NAC and WAC filter transmissions were measured as functions of wavelength for CCD temperatures ranging from -35C to about 26C. The source appeared in the data as a bright rectangle in the center of each image. After subtracting the dark model from the image values and performing smear corrections, a mask was derived from each image based on bright and dark pixel distributions. The values in the mask were set to zero for the dark regions and unity for the source rectangle. After multiplication, the pixel values for the resulting image were summed to achieve maximum response with varying source wavelength. For summed signal at each monochromator wavelength, the data were normalized to the highest sum, at the most sensitive wavelength. The center bandpass was solved assuming a square function, by summing the normalized signals times the delta wavelength range covered by each. The 50% point of the cumulative distribution was defined as the center wavelength. The derived center wavelengths and bandpasses are given above, and the relative filter sensitivities as a function of wavelength (used to model the radiance of the white integrating sphere as sampled by MDIS) are given in the CALIB directory. Conversion from radiance to I/F =============================== To convert from radiance to I/F (also known as radiance factor, the ratio of measured radiance to that which would be measured from a white perfectly Lambertian surface), the following expression should be applied: I_over_F(f) = L(f) * pi * (SOLAR_DISTANCE/149597870.691)**2 / F(f) where L(f) is calibrated radiance calculated as described above for some filter f, SOLAR_DISTANCE is that value for distance of the target object from the center of the sun in kilometers (as indicated by the keyword SOLAR_DISTANCE) 149597870.691 is the number of kilometers in 1 AU F(f) is effective average solar irradiance sampled under the filter bandpass. The effective average solar radiance for each camera and bandpass is given in the CALIB directory. Operational Considerations ========================== (1) WAC clear filter: Filter 2 on the wide-angle camera is broad-band and designed for star imaging. It is expected that typically even the minimum exposure time will saturate on Mercury. Therefore flat-field and responsivity corrections for WAC filter 2 were not derived. (2) NAC PSF. Due to mass constraints, the NAC aperture is smaller than what is required for diffraction-limited performance. The expected size of the Airy disk (approximately, the full-width at half-maximum of the point- spread function including only effects of diffraction) is > 2 pixels. In practice the PSF is further broadened by surface imperfections of optical elements and scatter centers on optical surfaces. (3) Compression. Wavelet compression applied to science images is typically lossy. Pre-launch, a study was done in which the appearance of artifacts at progressively higher wavelet compression ratios was investigated using raw images from the NEAR multispectral imager. Artifacts were identified by differencing raw images and the same images after compression and decompression. Spatially coherent artifacts were evident in monochrome images at compression ratios >8, and in color ratio images >4. Hence wavelet compression ratios of 8 and 4 were selected for nominal application to monochrome mapping and multispectral images respectively. (4) Frame transfer smear. At very short exposure times (<7 ms), the time for frame transfer is close to the total exposure so that the correction for frame transfer smear may leave perceptible artifacts. (5) Data Quality Index. A data quality index in the PDS label for each image is used to encode figures-of-merit into one parameter. The 16-byte data quality index includes automated assessments of validity of the exposure time, presence of an excessive number of pixels at or approaching saturation, validity of the reported pivot position, validity of the reported filter wheel position, quality of spacecraft attitude knowledge from the MESSENGER star cameras, CCD temperature within a well-calibrated range, and completeness of data within the commanded selection of subframes or full frame. (6) Temperature effects on attitude. The orientation of MDIS relative to the spacecraft reference frame was determined inflight using star calibrations to solve for WAC-NAC coalignment, the orientation of the pivot plane, and the origin of the reported pivot position within the plane. However these alignments can be affected by thermal state of the spacecraft. Mercury and Venus flybys are thermally benign. However in Mercury orbit the noon-midnight orbit experiences large thermal perturbations and errors in reported MDIS attitude of up to 350 microradians can be expected." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HAWKINSETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END